An important diagnostic of high-energy electrons accelerated in solar flares is the hard X-ray bremsstrahlung that they produce as they propagate through the ambient solar atmosphere. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI ) has revealed a new class of flares in which the bulk of the hard X-ray emission is produced predominantly not in dense chromospheric footpoints, but rather in the coronal loop [1]}, [2]}, [3]}. For such sources, the corona is not only the site of particle acceleration, but also dense enough to act as a thick target, stopping the accelerated electrons before they can penetrate to the chromosphere.
